In communication networks based on the Long Term Evolution (LTE) specifications promulgated by the Third Generation Partnership Project (3GPP), two radio frame structures are supported. A “type 1” structure is applicable to Frequency Division Duplexing (FDD) system configurations, where downlink (base station-to-user equipment) transmissions and uplink (user equipment-to-base station) transmissions take place in separated frequency bands. A “type 2” structure is applicable to Time Division Duplexing (TDD) system configurations, where downlink and uplink transmissions take place in different, non-overlapping time slots. In both frame structure types, each 10-millisecond radio frame is divided into two 5-millisecond half-frames, and each half-frame includes five 1-millisecond subframes.
Further, in frame structure type 2, each subframe is either a downlink subframe, an uplink subframe, or a special subframe. The various combinations of these subframe types in a frame give rise to different TDD configurations. The permitted configurations are depicted in Table 1, shown in FIG. 1. These configurations are defined in Table 4.2-2 of the 3GPP Technical Specification “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation,” 3GPP TS 36.211, v. 11.1.0 (December 2012), available at http://www.3gpp.org.
The supported uplink-downlink configurations in LTE TDD are listed in FIG. 1 where, for each subframe in a radio frame, “D” denotes a subframe that is reserved exclusively for downlink transmissions, “U” denotes a subframe that is reserved exclusively for uplink transmissions and “S” denotes a special subframe with the three fields: DwPTS (“downlink pilot time slot”), GP (“guard period”), and UpPTS (“uplink pilot time slot”). The guard time provided by the GP field is an important aspect of any TDD system, as it is required to avoid interference between uplink and downlink transmissions that might arise from propagation delays. The guard time also provides time for the equipment at each end of the link to switch between receive and transmit operations. The DwPTS and UpPTS parts of the special subframe are allocated to the downlink and the uplink, respectively. Depending on the configuration, the UpPTS is long enough to permit one or two OFDM symbols to be transmitted in the uplink, while the DwPTS can be long enough to permit as many as twelve symbols to be transmitted in the downlink. The length of DwPTS and UpPTS is given by Table 2, shown in FIG. 2, where Ts is the length of the fundamental period of the OFDM modulation and is equal to 1/(15000×2048) seconds. The length of GP can be determined from Table 2, subject to the total length of DwPTS, GP and UpPTS being equal to 1 millisecond. Each subframe consists of two slots, each of length 0.5 ms.
As seen in FIG. 1, each uplink-downlink configuration has either a 5-millisecond or 10-millisecond periodicity with respect to the downlink-to-uplink switch points. In configurations having a 5-millisecond downlink-to-uplink switch-point periodicity, the special subframe exists in both half-frames. In configurations having a 10-millisecond downlink-to-uplink switch-point periodicity, the special subframe exists only in the first half-frame. In all configurations, subframes 0 and 5 and the DwPTS portion of the special subframe are always reserved for downlink transmission. The UpPTS portion of the subframe and the subframe immediately following the special subframe are always reserved for uplink transmission.
In a TDD cell within a wireless communication network, a TDD configuration is characterized by both the uplink-downlink configuration and the special subframe configuration. Therefore the term “TDD configuration” used hereinafter refers to a combination of an uplink-downlink configuration configured in a TDD cell, e.g. one of the configurations depicted in Table 1, shown in FIG. 1, along with a special subframe configuration, e.g., one of the configurations depicted in Table 2, shown in FIG. 2. Of course, it will be understood that these are example configurations and that additional TDD configurations may be introduced in the future; thus, the teachings herein are not limited to these example configurations.
Dynamic TDD, e.g., dynamically changing TDD configurations, may be used to better adapt to changing network deployments and usage. For example, it is envisioned that there will be more and more localized traffic in the future, where most network users will be in hotspots, or in indoor areas, or in residential areas. The locations of these users will thus tend to be clustered and the aggregations of these users will tend to have different uplink and downlink traffic needs at different times. This circumstance essentially means that a dynamic feature to adjust allocations of uplink and downlink resources to instantaneous (or near instantaneous) traffic variations may be required in future local-area cells.
TDD is especially adapted to handle these varying traffic requirements, because the usable bandwidth can be configured in different time slots for use by either the uplink or the downlink. This approach allows for asymmetric allocations of uplink/downlink resources, which is a TDD-specific property, and not possible in FDD. As can be seen in FIG. 1, there are seven different uplink/downlink allocations in LTE, such that between 40%-90% of a given subframe's resources are allocated to the downlink. This can be more clearly seen in FIG. 3, which illustrates the allocation of uplink/downlink resources in each of the allowed TDD configurations. The configurations cover a wide range of allocations, from uplink-heavy, with a downlink-to-uplink ratio of 2:3 (Configuration 0) to downlink-heavy, with a downlink-to-uplink ratio of 9:1 (Configuration 5).
To avoid severe interference between downlink and uplink transmissions between different cells, neighbor cells in currently deployed LTE systems are typically configured with the same downlink/uplink configuration. If this is not done, uplink transmission in one cell can interfere with downlink transmission in the neighboring cell (and vice versa), as shown in FIG. 4. Hence, the downlink/uplink asymmetry in these systems is typically not varied between cells, but is signaled as part of the system information and remains the same for a long period of time
In these current networks, uplink/downlink configuration is semi-statically configured, which means that it may not match the instantaneous traffic situation. This mismatch results in inefficient resource utilization in both the uplink and downlink, especially in cells with a small number of users. Dynamic TDD addresses this issue by allowing a dynamic configuration of the TDD uplink/downlink asymmetry, to better match the current traffic situation and thereby optimize or at least improve the user experience. The dynamic TDD approach also can be utilized to reduce network energy consumption. Dynamic TDD is generally described in the 3GPP technical report “Further Enhancements to LTE Time Division Duplex (TDD) for Downlink-Uplink (DL-UL) interference management and traffic adaptation,” 3GPP TR 36.828, v2.0.0 (June 2012), available at http://www.3gpp.org.
Thus, the typical use of fixed TDD configurations in existing TDD networks, which semi-statically determines which subframes are uplink subframes and which subframes are downlink subframes, should be understood as limiting the ability to address changing uplink/downlink asymmetry arising from varying traffic situations. One approach to increasing TDD configuration flexibility, at least in some scenarios, is based on the idea that each subframe (or part of a subframe) belongs to one of three different types, downlink subframes, uplink subframes, or “flexible” subframes.
Downlink subframes or downlink subframe portions (i.e., DwPTS), are used (among other things) for transmission of downlink data, system information, control signaling, and hybrid-ARQ feedback in response to uplink transmission activity. This type of subframe is currently specified, i.e., per Release 8 of the specifications for LTE. During reception of a downlink subframe, a mobile terminal (a “user equipment,” or “UE,” in 3GPP terminology) monitors the Physical Downlink Control Channel (PDCCH) as specified in the LTE Release 8 standards, whereby it may receive scheduling assignments and scheduling grants. Special subframes are similar to downlink subframes except that, in addition to the downlink part (DwPTS), they also include a guard period (GP) as well as an uplink part (UpPTS) in the end of the subframe.
Uplink subframes are used (among other things) for transmission of uplink data, uplink control signaling (channel-status reports), and hybrid-ARQ feedback in response to downlink data transmission activity. This type of subframe is currently specified, i.e., per Release 8 of the specifications for LTE. Because the UpPTS is only one or two OFDM symbols in length, UpPTS usage is limited to either sounding reference signals or random access (RACH) transmission. Data transmission by a UE on the Physical Uplink Shared Channel (PUSCH) in uplink subframes or the UpPTS portion of a special subframe is controlled by uplink scheduling grants received on a PDCCH in an earlier subframe.
Flexible subframes, which are not specified in the LTE Release 8 standards, can be used for uplink or downlink transmissions, as determined dynamically by scheduling assignments/grants.
Semi-static configuration is used to assign one of the above four types (DL, UL, special, or FLEX) to each subframe. One example frame configuration under this approach is illustrated in FIG. 5, which shows a frame having flexible subframes (designated by “FLEX”) in the fourth, fifth, ninth, and tenth of the ten subframes. The semi-static configuration of subframe types may be accomplished by way of a Medium Access Control (MAC) Control Element or CE, or using Radio Resource Control (RRC) signaling, or by using a specific Radio Network Temporary Identifier (RNTI) on the PDCCH. Configuration information could be part of the system information as in Release 8 of the LTE standards, for example. The system information could explicitly indicate “UL”, “DL”, or “FLEX” for each subframe, for example. (The special subframe positions are implicitly indicated by the configuration of downlink and uplink subframes.) Alternatively, a configuration could be signaled by signaling “DL” and “UL” allocations for all of the subframes, using the conventional Release 8 signaling message, and then using an additional signaling message, understandable by new terminals only, that indicates that some uplink subframes should be configured as flexible subframes.
From the perspective of the UE, flexible subframes are treated in a similar way as downlink subframes, unless the UE has been instructed to transmit in a particular flexible subframe. Expressed differently, flexible subframes not specifically assigned for uplink transmission from a particular UE are, from a PDCCH perspective, treated as a downlink subframe. Hence, the UE generally monitors several candidate PDCCHs in a flexible subframe. If the control signaling indicates that the UE is supposed to receive downlink data transmission on the Physical Downlink Shared Channel (PDSCH), the UE will receive and process the PDSCH, just as it would for a downlink subframe. Similarly, if the control signaling contains an uplink scheduling grant valid for a later flexible subframe, the UE will transmit in the uplink accordingly.
In discussions and document of the dynamic TDD techniques summarized above, the terms dynamic TDD, flexible TDD, flexible UL/DL allocation, adaptive TDD, reconfigurable TDD, etc., may be interchangeably used. These terms all refer to the same concept. With dynamic TDD, one or more “dynamic” or “flexible” subframes can be used in different directions of transmission (i.e., uplink versus downlink) in different cells, which may belong to the same carrier or different carriers. Furthermore, the direction of flexible subframes in a particular cell can be changed over time, e.g., as fast as every radio frame. The controlling radio network node can decide whether and when to change the direction of a flexible subframe independently, or depending upon the TDD configuration used in one or more neighboring TDD cells. In principle, any subframe that is not adjacent to a special subframe can be configured as a flexible subframe. For example, in TDD configuration 0 (see FIG. 1), any of the subframes 3, 4, 8 and 9 can be configured as a flexible subframe.
An example of different TDD configurations across different cells is shown in FIG. 6. In the illustrated example, TDD Configuration 1 is used in the macro cell, as represented by the larger cell tower symbol, which may correspond to a macro LTE base station, known as an evolved Node B, or eNodeB. A flexible TDD configuration is used in the pico layer, as represented by the smaller cell tower symbol, which may correspond to a pico eNodeB. Note that the flexible subframes are indicated with an “F” in FIG. 6. It should be appreciated that base station-to-base station (BS-to-BS) interference can occur in certain subframes, depending on how the flexible subframes are used. This can have a serious impact on the uplink throughput in the victim cell, i.e., the cell that is trying to receive an uplink transmission from a UE while the neighboring cell is transmitting. Accordingly, while dynamic TDD has great potential for adaptively shifting uplink and downlink resources in response to traffic demands, further improvements are needed to ensure that excessive interference does not occur.